The present invention relates generally to a method and a device for determining shifting parameters to be used by at least a first and a second telecommunication devices for transferring symbols.
More precisely, the present invention is in the field of coding and decoding schemes used in the context of MIMO (Multiple Input Multiple Output) communications especially used in conjunction of OFDM or OFDMA-like transmission schemes.
Orthogonal Frequency-Division Multiplexing (OFDM) is based upon the principle of frequency-division multiplexing (FDM) and is implemented as a digital modulation scheme. The bit stream to be transmitted is split into several parallel bit streams, typically dozens to thousands. The available frequency spectrum is divided into several sub-channels, and each low-rate bit stream is transmitted over one sub-channel by modulating a sub-carrier using a standard modulation scheme, for example PSK, QAM, etc. The sub-carrier frequencies are chosen so that the modulated data streams are orthogonal to each other, meaning that cross talk between the sub-channels is eliminated.
The primary advantage of OFDM is its ability to cope with severe channel conditions, for example, multipath and narrowband interference, without complex equalization filters. Channel equalization is simplified by using many slowly modulated narrowband signals instead of one rapidly modulated wideband signal.
A variation called DFT spread OFDM or SC-FDMA (Single Carrier Frequency-Division Multiple Access) has been developed. In this system each symbol to be transmitted is spread over a set of transmitted frequencies by a DFT (Discrete Fourier Transform), the resulting signal is sent over a conventional OFDMA transmission system.
Actual implementation of coding/decoding are made either in the frequency domain or in the time domain while the implementation in the frequency domain may be preferred.
It is known that the use of several antennas both at the emitter and the receiver, leading to MIMO systems allows the improvement of the robustness of the transmission. This improved robustness can be used to increase the range or the bandwidth by adjusting the classical range versus bandwidth tradeoff. Several types of diversity schemes could be used to take advantage of multiple antennas at the emitter.
Alamouti has developed an Orthogonal Space Time Block Code (OSTBC) wherein information to be transmitted are spread in space, by the different antennas, and in time, using different time slots. The reference paper regarding Alamouti codes is “A simple transmit diversity technique for wireless communications”, IEEE J. Select. Areas Commun., vol. 16, pp. 1451-1458, October 1998. In a first implementation of Alamouti code, two transmit antennas (FirstAnt and SecondAnt) are used for transferring two symbols a and b in two time slots (T1 and T2). At time T1 antenna FirstAnt transmits symbol a when antenna SecondAnt transmits symbol b. At time T2 antenna FirstAnt transmits symbol −b* when antenna SecondAnt transmits symbol a*, where “*” denotes the complex conjugate. This Alamouti code, let us call it classical Alamouti in time, has the advantage to offer simple coding and decoding, the increased diversity leading to better performance. It is to be noted that the throughput is not increased. The optimal MAP for Maximum A Posteriori decoding is very simple, it does not imply matrix inversion, log enumeration or sphere decoding as long as the channel does not vary between T1 and T2 and as long as the channel can be characterized by a simple multiplication. It is naturally well combined with OFDM or OFDM-like modulation schemes.
A second implementation of Alamouti code called OSFBC for Orthogonal Space Frequency Block Code is based on transmission of the data over two different frequencies (F1 and F2), and not over two different time slots. With two transmit antennas (FirstAnt and SecondAnt), two symbols a and b are respectively sent on two frequencies (F1 and F2) using an antenna FirstAnt transmits symbol a when antenna SecondAnt transmits symbol b. Through the antenna FirstAnt, the symbol −b* is sent on the frequency F1 and the symbol a* is sent on the frequency F2 through the antenna SecondAnt.
The two frequencies are adjacent, to limit the variations of the channel.
By definition, this scheme is applied to OFDMA or OFDMA-like modulation schemes. By OFDMA-like modulations, we denote for example some frequency-domain implementation of a single carrier scheme, in which preferably, but not strictly necessarily, a cyclic prefix has been added, like for example the described DFT-spread OFDM. Compared to OSTBC, the advantage is the use of only one modulation slot, which can be advantageous from the multiplexing point of view, and may lead to better performance in case of very fast variations of the channel like high Doppler. Alamouti codes, due to their simple implementation and good performance are very attractive schemes to be used in MIMO transmission. When applied to SC-FDMA like modulation schemes, these codes do not have the valuable feature to produce signals with the low variation envelope property for each antenna, the envelope being the modulus of the complex envelope.
In the published patent invention WO 2008/098672, it has been proposed a method of radio data emission, by an emitter comprising at least two transmit antennas. The signal transmitted on a first antenna being considered in the frequency domain as resulting from a DFT of size M leading to the emission of a symbol on each of the M sub carriers allocated to the emitter on the first antenna. A SC(p) relation is defined by SkSecondAnt=(−1)k+1S*(p-1-k)mod M for k=0 to M−1 giving the signal to be emitted on a second antenna SecondAnt from the signal S to be emitted on the first antennaFirstAnt, where p is an even shifting parameter between 0 and M−1 and k is the index of each used sub carrier in the frequency domain.
The use of above mentioned technique is not adapted into systems wherein plural devices like mobile stations use different bandwidths for data transmission which overlap each other.
The FIG. 1a shows an example wherein a first emitter comprises two transmit antennas Ant11 and Ant12. The signal transmitted on the antenna Ant 11 being considered in the frequency domain as resulting from a DFT of size M=8 leading to the emission of a symbol on each of the M sub carriers on the antenna Ant 11. The SC(p) relation, with p=4, defined by X′Ant12k=(−1)k+1*(p-1-k)mod M for k=0 to M−1 gives the signal to be emitted on the antenna Ant12 from the signal X to be emitted on the antenna Ant11.
The FIG. 1b shows an example wherein a second emitter comprises two transmit antennas Ant21 and Ant22. The signal transmitted on the antenna Ant21 being considered in the frequency domain as resulting from a DFT of size M=12 leading to the emission of a symbol on each of the M sub carriers on the antenna Ant 21. The SC(p) relation, with p=6, defined by Y′Ant22 k=(−1)k+1Y*(p-1-k)mod M for k=0 to M−1, gives the signal to be emitted on the antenna Ant22 from the signal Y to be emitted on the antenna Ant21.
When the first and second emitters transmit simultaneously data on frequency bands which overlap, for example when the first emitter transmits data on the frequency band composed of sub-carriers noted 1 to 8 of the FIG. 1c and the second emitter transmits data on the frequency band composed of sub-carriers noted 0 to 11, some impairment problems occur. On the sub-carrier 5, the couples of data (X4, −X7*) and (Y5, Y0*) are transferred but on the carrier 8, the couple of data (X7, X4*) is transferred and the couple (Y0, −Y5*) is not transferred on that carrier.
Such impairments lead to situation wherein the decoding of the received symbols at the receiver side is not possible.